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12606B
Section IV
Table 4-3. Command Card Flip-Flops and Registers (Continued)
FLIP-FLOP OR REGISTER
FUNCTION
Track Origin Store
Reset by a CLF instruction which addresses the command card. Set when the track origin
(TOS) FF
is reached. Remains set until cleared by another CLF instruction.
Word Counter
Counts words transferred to or from the disc. Flip-flop WDO contains the low-order bit.
The counter is reset by the second "not" SC pulse of each sector. The counter runs only
during transfer of data to or from the disc.
Word (WRD) FF
When transferring words to or from disc, indicates when the end of each word is reached.
Set at the start of each word transferred, reset near the end of each word.
4-31.
As well as clearing the Run FF, the "not" CRF
signal clears the sector address register (flip-flops S6
through SO) on the command card. This is done through
MC14A, which functions as a negative-logic "nor" gate.
4-32.
When the power ON switch on the disc power
supply is set to the ON position, the disc starts to rotate
and the disc memory supplies a true "not" RY signal to the
command card until the disc reaches operating speed. The
"not" RY signal is inverted on the command card, and it
clears the SAC FF and sets the ABS FF.
4-33.
If
the disc is not connected to the computer, the
"not" RY input pin to the command card faces an open
circuit. The open input to inverter MC65A produces the
same result as if the "not" RY signal were true. Other faults
which will keep the "not" RY signal true are the following:
a. The circuit in the disc memory which supplies the
"not" RY signal is defective.
b. The disc memory is not connected to the disc
power supply.
c. No line voltage is applied to the disc power supply.
d. The disc power supply is defective or not turned on.
4-34.
When the disc is ready for use, the "not" RY
signal becomes false. Through "and" gate MC67 A on the
command card, the inverted "not" RY signal can be gated
onto the lOBI7 line. This occurs when an LIA/B instruc-
tion addresses the command card. The instruction places all
lOBI bits from the command card into the A- or B-register
in the computer. These bits constitute the disc status word,
and bit 7 will indicate whether the disc is ready for use.
(Refer to table 3-2.)
4-35.
After the power tum-on period the sector counter,
on the command card, assumes an unpredictable condition.
(Flip-flops SC6 through SCO make up the sector counter.)
The first "not" TO pulse received from the disc resets the
counter to zero, after which it functions in synchronism
with the "not" SC pulses received from the disc.
Figure 4-2 is a timIng chart showing the operation of the
counter and its associated SCP FF.
4-36.
It
should be noted that the SCP and SC6 through
SCO FFs are of the dual-rank JK type. The positive-going
clock input to each flip-flop gates the signal input into the
first rank of the flip-flop. The negative-going clock input
transfers the original signal input to the output rank of the
flip-flop.
4-37.
Each pair of "not" SC pulses advances the sector
counter by 1, with the SCP FF functioning as a divide-
by-two counter preceding the sector counter. The change of
the output rank flip-flops in the sector counter takes place
at the trailing edge of the second "not" SC pulse of each
pair.
4-38.
The first pair of "not" SC pulses after the "not"
TO pulse advances the counter to 00000001, binary. This
number is one greater than the address of the sector that is·
about to come under the read-write heads. The counter
continues to function in this manner for each sector in the
track. When the last sector has passed, the "not" TO pulse
clears the counter.
4-39.
A voltage transient or momentary equipment
failure could result in incorrect contents in the sector
counter and the SCP FF.
If
this occurs, the next "not" TO
pulse will return the counter and SCP FF to their proper
state, restoring normal operation. Every "not" TO pulse
sets the TOS FF.
4-40.
The address of the next disc sector can be gated
onto the IOBI14 through 8 lines, and placed in the A- or
B-register in the Computer, by an LIA/B instruction that
addresses the command card. (Refer to table 3-2, bits 14
through 8.)
4-41.
The set state of the SCP FF indicates that the
sector counter is about to be advanced. This advance occurs
when the SCP FF is reset. The condition of the SCP FF can
be sampled by an LIA/B instruction which addresses the
command card. When this instruction is executed, the lOBI
outputs from the command card are loaded into the
computer A- or B-register as the disc status word, and
bit 15 of the status word can then be examined by the
program to determine the state of the SCP FF. (Refer to
table 3-2.)
If
the SCP FF is set (bit 15 of the status word is
logic 1, the next sector is about to pass under the read/
4-5
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